Safety Loop Redundant Solenoids for Exercise and Testing

Solenoid valves are often found in safety instrumented functions (SIFs) to release pneumatic pressure holding safety valves open or open depending on the application. Redundancy of these valves enables greater reliability and online proof testing.

In a Process Industry Informer article, Redundant Solenoid Valve Architecture Improves Safety and Reliability, Emerson’s Mike Howells describes the importance of these valves for safety, and architectures to exercise and test these valves for ongoing reliable performance.

Redundant architecture for safety instrumented function

Mike opens by highlighting the importance of redundancy.

By allowing facilities to perform critical solenoid valve testing online, redundant architectures help keep personnel and equipment safe while reducing downtime.

A safety instrumented function may go years without ever activating to bring the process to a safe state. It’s essential to make the solenoid valves operate.

If it goes long stretches without activity, the solenoid valve can experience stiction. Stiction is a lot like it sounds: It’s the sticky friction that can keep stationary surfaces in contact with one another from moving apart. Periodically exercising valves both prevents stiction and verifies it isn’t there to begin with.

An architecture with redundancy enables testing.

A two- or three-solenoid safety methodology enables online valve testing and maintenance, providing greater protection and improving uptime while complying with functional safety requirements.

Collectively, the SIFs make up the safety instrumented system (SIS).

An SIS typically consists of sensors, logic solvers or controllers and final elements. Sensors measure process parameters, such as pressure, temperature, flow, level and gas concentration. Logic solvers or controllers translate signals from sensors and complete preprogrammed actions to prevent or alleviate process dangers. Final elements, which include the ESD [emergency shutdown] valve and a pneumatic, electric or hydraulic actuator and solenoid valve, bring the process to a safe state.

Existing SIFs can be retrofitted for redundancy.

Installing a second solenoid valve isn’t particularly time-consuming or even complicated. In fact, there is a prepackaged solution that uses redundant solenoid configurations. The redundant control system (RCS) is a proven pilot valve arrangement that provides built-in redundancy and diagnostics.

The ASCO Series RCS Redundant Control System can perform automated testing.

The RCS’s automated online testing, including solenoid valve and partial stroke tests, allows users to identify 98% of hazardous failure points with no bypassing, while pressure switches provide continuous monitoring and diagnostic feedback. When the bypass is activated, the RCS also makes online maintenance, such as replacing solenoid valves, coils and pressure switches, fast and easy with no process interruption.

Read the article for more on the various architectures, including 2oo2D [2 out of 2 with diagnostics to detect failure conditions], 2oo3, and 2oo3D, and how the RCS performs this testing in safety applications up to SIL 3. Mike also shares how the different architectures affect the probability of failure on demand (PFD) and spurious trip rate (STR) for the various architectures—1oo1, 1oo2, 2oo2, 2oo3, 2oo2D, and 2oo3D.

Visit the ASCO Series RCS Redundant Control System page on for more on this pilot valve system with no single point of failure and is suitable for use in up to SIL 3 safety applications.

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